1. Field of the Invention
This invention relates to an improved additive for use in cementitious compositions. More particulary, this invention relates to an additive comprised of a cellulose ether, a starch ether, and a polyacrylamide. The additive can be added to a cementitious composition to improve the properties thereof.
2. Background of the Related Art
U.S. Pat. No. 4,021,257 to Bernett discloses a cementitious composition which comprises 25 to 100% Portland cement and from 0.001 to 0.25% of certain long-chain organic polymers having a Flocculation Index of less than 0.5. Among the long-chain organic compounds disclosed in the patent are polyacrylamides and alum hydroxyalkyl starch. The examples of the patent are limited to the use of either a hydroxyalkyl starch or a polyacrylamide.
U.S. Pat. No. 4,447,267 to Chesny, Jr. et al. discloses dry-set Portland cement based grout compositions having less than 75% by weight Portland cement, filler, bentonite clay, and no more than about 0.35% by weight of a composite of a cellulose ether and a long-chain polymer such as polyacrylamide. The patent discloses that the preferred embodiments of that invention are improved grout compositions over the previous grouts developed in U.S. Pat. Nos. 4,021,257 to Bernett and 4,043,827 to Bernett.
U.S. Pat. No. 3,847,630 to Compernass et al. discloses a method for the production of a water-permeable porous concrete in which a macromolecular water-soluble compound is employed either a plant gum, starch, cellulose ether or a synthetic polymer such as polyacrylamide.
U.S. Pat. No. 4,363,667 to Birchall et al. discloses a cementitious composition in which at least one polymeric water soluble or water dispersible additive is employed. Such additive includes cellulose ethers; amide containing polymers such as copolymers of acrylamide; polyoxyalkylene oxide derivatives or hydrolyzed vinyl acetate polymers.
U.S. Pat. No. 4,082,563 to Ellis et al. discloses a mortar composition containing at least one water soluble high molecular weight polymer, such as cellulose ethers or polymers of polyvinyl alcohol or polyacrylamide.
There is a continuing need in the art for a cementitious composition which has improved anti-sag characteristics and yet which also has a long open time, i.e. a composition which will adhere to and support a tile when it is set in the composition but which will not set so fast so as to preclude either a long interval between application of the cementitious composition and placement of the tile or the adjustability of the tile once initially placed in the composition.